Lead extraction from Cathode Ray Tube (CRT) funnel glass: Reaction mechanisms in thermal reduction with addition of carbon (C)

Waste Management xxx (2018) xxx–xxx

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Lead extraction from Cathode Ray Tube (CRT) funnel glass: Reaction mechanisms in thermal reduction with addition of carbon (C) Xingwen Lu a,b, Xun-an Ning a, Da Chen d, Kui-Hao Chuang c, Kaimin Shih b, Fei Wang b,d,⇑ a

School of Environmental Science and Engineering and Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, China Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region c Department of Safety Health and Environmental Engineering, Central Taiwan University of Science and Technology, Taichung 406, Taiwan, ROC d School of Environment, Guangzhou Key Laboratory of Environmental Exposure and Health, and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, China b

a r t i c l e

i n f o

Article history: Received 3 January 2018 Revised 27 March 2018 Accepted 6 April 2018 Available online xxxx Keywords: Cathode Ray Tube (CRT) Funnel glass Thermal reduction Quantitative X-ray diffraction Lead recovery

a b s t r a c t This study quantitatively determined the extraction of lead from CRT funnel glass and examined the mechanisms of thermally reducing lead in the products of sintering Pb-glass with carbon in the preheated furnace. The experimentally derived results indicate that a 90.3 wt% lead extraction efficiency can be achieved with 20 wt% of C addition at 950 °C for 3 min under air. The formation of viscous semi-liquid glass blocked the oxygen supply between the interaction of C and Pb-glass, and was highly effective for the extraction of metallic Pb. A maximum of 87.3% lead recover was obtained with a C to Na2CO3 ratio of 1/3 at 1200 °C. The decrease of C/Na2CO3 ratio enhanced the metallic lead recovery by increasing the glass viscosity for effective sedimentation of metallic lead in the bottom. However, with the further increase of temperature and treatment time, re-vitrification of lead back to silicate-glass matrix was detected in both Pb-glass/C and Pb-glass/C/Na2CO3 systems. The findings indicated that with proper controls, using C as an inexpensive reagent can effectively reduce treatment time and energy, which is crucial to a waste-to-resource technology for economically recovering lead from the waste CRT glass. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Cathode Ray Tubes (CRTs) have been extensively used for more than 70 years. The number of discarded CRTs in televisions and computer monitors has drastically increased because of the replacement of CRT by advanced flat panel products (Chen et al., 2010). In North America, approximately 163,420 computers and televisions became obsolete every day in 2006, and the cost of proper disposal or recycling reached as high as $10.8 billion by the year 2015 (Kim et al., 2009). In Europe, 50,000–150,000 tons of end-of-life CRTs were collected each year, and this flux is not expected to decrease in the upcoming years (Singh et al., 2016a). In China, more than 32 million units of waste televisions and 37 million units of waste computers, as well as 43.11 million tons of CRT glasses were generated in 2013 only (Singh et al., 2016b).

⇑ Corresponding author at: School of Environment, Guangzhou Key Laboratory of Environmental Exposure and Health, and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, China. E-mail address: [email protected] (F. Wang).

Additionally, approximately 6 million discarded TVs and 10 million computers are stockpiled each year (Xu et al., 2012). Waste CRTs are mostly generated from panel and funnel glasses accounting for about two-thirds of the whole weight of the device (Andreola et al., 2005). The front part of CRTs is called panel glass, which is made of barium-strontium glass free of lead (Andreola et al., 2005; Musson et al., 2000). The rear part of the CRTs is called funnel glass, made of silica and lead oxide (
20 wt% of PbO) (Musson et al., 2000; Andreola et al., 2007). CRTs are usually incinerated and dumped in landfills as municipal solid waste (MSW) in most countries, even developed countries (Jang and Townsend, 2003; Tian and Wu, 2016; Needhidasan et al., 2014). Unfortunately, lead can leach out from the landfilled CRT funnel glasses, which may seriously contaminate the environment and endanger human health (Liu et al., 2011; Turbini et al., 2001). Thus, the development of new technology to detoxify and recycle CRT glass is strongly needed. Generally, closed-loop and open-loop recycling are two principal ways of recycling CRT glass (Singh et al., 2016a; Herat, 2008). In closed-loop recycling, waste CRT glass is commonly used as raw material to manufacture new CRTs (Herat, 2008; Gregory

https://doi.org/10.1016/j.wasman.2018.04.010 0956-053X/Ó 2018 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Lu, X., et al. Lead extraction from Cathode Ray Tube (CRT) funnel glass: Reaction mechanisms in thermal reduction with addition of carbon (C). Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.04.010

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X. Lu et al. / Waste Management xxx (2018) xxx–xxx

et al., 2009). However, the increasing supply of secondary glass has exceeded the continuously declining demand of new CRT glass as a result of their replacement by new display technologies (Singh et al., 2016a; Herat, 2008; Gregory et al., 2009; Kang and Schoenung, 2006). Some applications have been proposed for the use of recycled CRT glass in the manufacture of clay bricks, decorative tiles, nuclear waste encapsulation, foam glass, ceramic glaze and fluxing agent for open-loop recycling (Ling and Poon, 2012; Dondi et al., 2009; Ling and Poon, 2011; Andreola and Barbieri, 2007). However, a limitation of these techniques is the potential threat to environmental and human health associated with leadcontaining products. Development of techniques to extract lead from funnel glass is essential to safe disposal of waste CRTs (Singh et al., 2016c). Many technologies have been developed to extract lead from CRT funnel glass, including pyrometallurgy, mechanochemical method, enhanced leaching and others (Singh et al., 2016a; Tian and Wu, 2016; Singh et al., 2016c). Operational parameters and product properties of various methods used for extracting lead from CRT funnel glass are summarized in Table 1. Among the advanced lead extraction technologies, pyrometallurgy was proven to be an effective way to extract lead from Pb-glass matrix (ICER, 2003; Lu et al., 2013; Xing and Zhang, 2011; Chen et al., 2009; Okada and Yonezawa, 2013; Okada, 2015; Okada et al., 2015; Lv et al., 2016; Veit et al., 2015). Since lead containing PbO3, PbO4, and PbO6 polyhedra are firmly encapsulated between SiO4 tetrahedrons to form

the glass network (Méar et al., 2006a, 2006b), the extraction of lead from the complex network structure often involves high energy consumption to break the structure of lead silicate and separate lead from funnel glass. However, the usage of reducing agent can change the structure of three-dimensional glass and extract Pb from PbOx polyhedron via redox reaction at relative low temperature with less time consumption. Thus, the development of costeffective processes is necessary and meaningful to recover Pb from CRT funnel glass for environmental protection and resource recovery. This work used C as a reducing reagent to extract the metallic lead from glass matrix at temperatures of 600–1000 °C in air. The sintering conditions (temperature, holding time, reagent dosage) were optimized to find the proper parameters for lead extraction. Furthermore, Na2CO3 was used to recovery lead from CRT funnel glass. Reaction mechanisms in thermal reduction with addition of C and Na2CO3 were examined by quantitatively determining the phase transformation during the reductive reaction of CRT funnel glass with C. 2. Materials and methods 2.1. Materials Funnel glass from color computer monitors was collected in Hong Kong and crushed into small pieces (around 3 cm) with the

Table 1 Operational parameters and product properties of various methods used for extracting lead from CRT funnel glass. Technology

Reagents

Temperature (C)

Pressure

Time

Extraction ratio (ER, %) or Recovery ratio (RR, %)

Product Properties

Pyrometallurgy

Al (ICER, 2003) SiC/TiN (Yot and Méar, 2009)

1400 950

1 atm 1 atm

ER, 50 ER, 40

Metallic lead, glass Foam glass, metallic lead

Mg, Fe2O3 (Wang and Zhu, 2012)

>2000

1 atm

ER, 90

Fe (Lu et al., 2013)

700

1 atm

Refractory materials, lead oxide Metallic lead, glass

Poly(vinyl chloride), Ca(OH)2 (Grause et al., 2014a, 2014b) CaCl2 (Erzat and Zhang, 2014)

1000

1 atm

1.5 h 1–4 h 1 min 30 min 2h

1000

2h

RR, 99.1

C (Xing and Zhang, 2011; Chen et al., 2009)

1000

1h

RR, 96

Porous residue, nano-lead

C, Na2CO3 (Okada and Yonezawa, 2013; Okada, 2015; Okada et al., 2015) C, lead slag (Lv et al., 2016)

1000

600 ± 50 Pa 500– 2000 Pa 1 atm

PbCl2, calcium silicatecontaining residues PbCl2

RR, 92

Metallic lead

1200

1 atm

ER, 97

C (Veit et al., 2015) 2-[Bis(carboxymethyl)amino], acetic acid, ferric sulfate (Singh et al., 2016b) Na2EDTA, ferric sulfate (Sasai et al., 2008)

800 Room temperature Room temperature Room temperature 355

1.3 kPa, 1 atm

60 min 60 min 18 h 12 h

ER, 92 RR, 99.8

Metallic lead, calcium silicateslag N.A. PbSO4, silica powder

1 atm

20 h

RR, 99

PbSO4, silica powder

1 atm

2h

ER, 92.5

Glass, PbS

24 MPa

ER, 93

Room temperature 37

1 atm

18– 22 h 1h

ER, 90

Crystalline alkaline-earth lead silicate, orthoclase Pb2+

12 days 1h

N.A.

Pb-EDTA

ER,100 and RR 43.3

sodium silicate, PbS, PbSO4

30 min 30 min

ER, 94.8

Pb2+, SiO2 glass

ER, 99.8

Pb2+, SiO2 glass

Mechanochemical treatment

S (Yuan et al., 2012, 2013, 2015) Enhanced leaching

Nitric acid (Miyoshi et al., 2004) Nitric acid, ultrasound (Saterlay et al., 2001)

Thermal reduction + acid leaching

Serratia plymuthica and EDTA (Pant et al., 2014) Na2CO3, Na2S (Hu et al., 2015)

1300

1 atm

C, HNO3(5 mol/L) (Xing et al., 2016)

1200

0.75 MPa 1 atm

B2O3, HNO3(5 mol/L) (Xing et al., 2017)

1000

1 atm

ER, 58 ER, 99.9

N.A. Not avaiable. ER: Extraction ratio (%). RR: Recovery ratio (%).

Please cite this article in press as: Lu, X., et al. Lead extraction from Cathode Ray Tube (CRT) funnel glass: Reaction mechanisms in thermal reduction with addition of carbon (C). Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.04.010

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coating fully removed by the wet scrubbing method (Lee and Hsi, 2002). The cleaned funnel glass particles were then dry ball milled and sieved to collect particles smaller than 80 lm. The powder obtained was dried at 105 °C for 24 h. The chemical composition of the glass powder was examined by XRF (JSX-3201z, JEOL) to be SiO2 (51.72%), PbO (21.50%), K2O (8.88%), Na2O (5.70%), CaO (3.63%), Al2O3 (3.03%), MgO (2.98%) and others (2.55%). Carbon powder with a particle size of 4–8 lm was obtained from Aldrich for use as the reducing agent. Na2CO3 was also purchased from Aldrich and used as the fluxing agent in the thermal reduction process. 2.2. Thermal reaction The experiments were carried out by heating the mixture of waste funnel glass and reducing agent C with or without fluxing agent Na2CO3 in air atmosphere. Samples weighed with different ratios of funnel glass, C and Na2CO3 were homogenized by ball milling. The well-mixed powders were transferred into a 30-mL alumina crucible and combusted in a high temperature furnace. The powders were heated at 600–1200 °C with temperature increasing rates of 5, 10, 20, 30 and 40 °C/min. Powder mixtures were directly put into the pre-heated furnace. The door of the furnace was opened slightly in a fraction, and the solid sample was quickly placed into the furnace before the temperature dropped. The retention time of thermal treatment in the furnace was controlled at 3–120 min under air condition. After thermal treatment, the samples were allowed to cool naturally to room temperature. After quenching, the solid samples were transferred from the crucible onto weighing papers for weight analysis. Because the Rietveld quantitative analysis can only determine the phase composition of each chemical in the mixture, the product weight of each sample would be used to calculate the actual weight of each compound. Finally, the samples were ground into powder with a particle size of less than 10 lm for XRD analysis. 2.3. X-ray diffraction

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corresponding reliability values, indicating the good refinement quality achieved using this analytical scheme, are provided in SI Tables S1–S4. 3. Results and discussion 3.1. Effect of heating rate To investigate the influence of heating rate on lead extraction, a mixture of CRT Pb-glass and C with a Pb-glass/C mass ratio of 1/0.1 was heated with a temperature increasing rate of 5, 10, 20, 30 or 40 °C/min, respectively, and in the pre-heated furnace at 950 °C for 3 min under air atmosphere. As shown in Fig. 1, the signal of metallic Pb was significantly increased with the increase of temperature heating rate. Glass transition temperature (Tg) of CRT glass was reported at around 480 °C (Méar et al., 2006a, 2006b); thus the Pb-glass quickly melts into a viscous semi-liquid at 950 °C in the pre-heated furnace. This highly viscous matrix blocks the oxygen supply during the interaction between C and Pb-glass, and is highly effective for inhibiting C oxidation and metallic lead extraction. Thus, strong signals of Pb were observed in the XRD pattern of thermally treated product. In contrast, the peak intensity of Pb was low in the products heated with a slower heating rate (5, 10 and 20 °C/ min) due to the slower transformation of solid to liquid, which could not prevent the oxidation of C. Without the inhibition of oxygen in the air, the C would be quickly combusted other than reducing Pb in PbOx polyhedra into metallic lead. To evaluate the lead extraction efficiency with different heating rates, quantitative phase compositions of the heated products were determined by diffraction data and Rietveld analysis. Using the results of the quantitative XRD analysis, an extraction ratio (ER) index of metallic lead was defined by Eq. (1) to indicate the extraction efficiency:

ERð%Þ ¼

    MW Pb =ðmCRT  wt0Pb Þ  100% mT  wtPb þ wtPbO  MW PbO

ð1Þ

Phase transformation during thermal reduction process was identified by the X-ray powder diffraction technique. The X-ray diffraction data from the powder samples were collected on a Bruker D8 Advance X-ray powder diffractometer equipped with a Cu Ka radiation and a LynxEye detector. The diffractometer was operated at 40 kV and 40 mA, and the 2h scan range was 10° to 80° with a step size of 0.02° and a scan speed of 0.3 s/step. Phase identification was performed using Eva XRD Pattern Processing software (Bruker Co. Ltd.) by matching the powder XRD patterns with those retrieved from the Powder Diffraction File (PDF) database published by the International Centre for Diffraction Data (ICDD). The crystalline phases found in the products included lead (Pb; PDF #65-2873), lithargite (PbO; PDF #05-0561), and aluminum (Al; PDF #04-0787). 2.4. Rietveld refinement A quantitative refinement method using 25% CaF2 as the internal standard (De La Torre et al., 2001; Magallanes-Perdomo et al., 2009; Rendtorff et al., 2010) was used to quantify the amorphous content in the samples. XRD scans were conducted from 2h = 10° to 110°, with a step width of 2h = 0.02° and a sampling time of 0.5 s per step. The Rietveld refinements for phase quantification were processed by the TOPAS (version 4.0) program. Structure models of the crystalline phases were obtained from the ICDD database. Figs. S1–S3 in the Supplementary Information (SI) present the Rietveld refinement plots of the products sintered from the C/Pb-glass and Na2CO3/C/Pb-glass systems, and the

Fig. 1. XRD patterns of the Pb-glass/C of 1/0.1 mixtures thermally treated in heating rates of 5, 10, 20, 30 and 40 °C/min, and the pre-heated furnace at 950 °C for 3 min under air atmosphere.

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Fig. 2. Effect of the heating rate on the Pb extraction efficiency. The extraction ratio (ER) values are plotted for the mixtures with mass ratio of Pb-glass/C of 0.1/1 at 950 °C for 3 min.

where mT is the weight of the treated products; wtPb and wtPbO are the amount of metallic lead and PbO in the treated sample, respectively; MWPb is molecular weight; mCRT represents the weight of the CRT funnel glass used for the treatment; and wt’Pb is 19.9 wt% based on the XRF result . Fig. 2 summarized the ER values of C on CRT glass as the function of heating rate at 950 °C for 3 min. Substantial increase of Pb extraction efficiency was observed with the increase of heating rate, and the lead extraction ratios increased significantly to 65% in the heating product. This finding clearly demonstrated that a pre-heated atmosphere was the optimized condition for lead extraction. The significant formation of metallic lead in the preheated furnace was probably due to the quick fusion of the mixtures into a viscous semi-liquid at 950 °C. Such a reaction would block the oxygen supply during the interaction between C and effectively inhibit the oxidation of C. The thermal reduction of lead in the glass with addition of C can be revealed by the reaction equation described in Eq. (2):

BSiAOAPbA + AOA + C(0) ! Pb(0) + BSiAOA + ACAOA

ð2Þ

In the reductive melting process, C reacts with O- in 3dimensional viscous network for reducing Pb2+ in glass networks. The result also shows the benefit of using this new method for industrial operations, because the continuous processing (materials directly entering the constant-temperature zone without slowing temperature ramping) and short reaction times are usually preferred. 3.2. Effect of temperature, treatment time and C dosage Extraction of lead from CRT funnel glass was reported to be strongly dependent on treatment parameters (Lu et al., 2013; Xing and Zhang, 2011; Chen et al., 2009; Okada and Yonezawa, 2013; Okada, 2015; Okada et al., 2015; Lv et al., 2016). To obtain the optimal parameters for the efficient recovery of lead, different doses of the reducing agent (C) were tested under temperatures of 600–1000 °C for 3–120 min. Fig. 3(a) summarizes the obtained ER values as the function of temperature with a Pb-glass/C mass ratio of 1/0.1. At temperatures of 600–950 °C, substantial increases of Pb extraction efficiency were observed and the lead extraction ratios increased significantly to 65%. However, the ER value dramatically decreased when temperature increased from 900 °C to 1000 °C. The results of the quantitative XRD revealed the most effective

Fig. 3. Effect of temperature, treatment time and Pb-glass/C mass ratio on Pb extraction efficiency. Extraction ratio (ER) values were obtained for Pb-glass/C samples with different mass ratios thermally treated at 600–1000 °C for 3–120 min.

temperature for lead extraction from glassy networks was around 950 °C. The significant growth of metallic Pb at 950 °C is probably due to the intensive lead diffusion across the glass-C interface that occurs in the crystallization of metallic lead (Rawlings et al., 2006), together with the higher mobility of C in the viscous glass at higher temperatures (Kushima et al., 2009). However, the dramatic decrease at a higher temperature (1000 °C) may be attributed to the potential transformation of the lead crystals back to the glassy matrix or the volatilization of metallic lead at higher temperatures. To evaluate the metal evaporation under the test conditions, samples with a Pb-glass/C mass ratio of 1/0.1 were weighed before

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and after thermal treatment at 800–1000 °C and were subjected to elemental characterization using the X-ray fluorescence (XRF) technique (Table S5 SI). According to the sample weight and XRF data, the loss of Pb due to evaporation was determined to be less than 1 wt%. This finding indirectly indicates that the amorphization of lead occurs at higher temperatures (1000 °C or above), suggesting the extracted lead transforms back to the glass product. Fig. 3(b) summarizes the quantified results of the lead extraction efficiency as a function of treatment time of 3–120 min. At 700 °C, the lead extraction efficiency initially increased before 30 min of heating, but then gradually decreased with a prolonged heating time. At higher temperatures of 950 and 1000 °C, negative relationships between the lead extraction efficiency and thermal treatment time were found. The findings suggested two key reaction mechanisms of lead during the thermal reduction process: the reduction of lead by C with a short treating time as described by Eq. (2) and the re-vitrification of lead back to silicate-glass matrix with a prolonged treating time as defined by Eq. (3).

BSiAOASiB + Pb(0) ! BSiAO Pbþ +  SiB

ð3Þ

Similar observation was previously reported (ICER, 2003; Lu et al. 2013), which suggest that the incorporation of lead into the glass matrix is due to the oxidation of lead under a long reaction time and further melting. The lead extraction pathways during the extraction process were shown in Fig. 4. Pathway I represents the main reactions for extraction metallic lead from Pb-glass and the formation of crystal lead at 600–950 °C for 3 min heating; pathway II shows higher temperatures (950–1000 °C) and longer heating time initiate the re-vitrification of lead back to silicateglass matrix. Fig. 3(c) illustrates the lead extraction with different doses of the reducing agent under a treatment of 950 °C for 3 min. The results revealed that lead extraction was generally enhanced by an increase in C. At 950 °C, the lead extraction efficiency increased with the increasing Pb-glass/C weight ratio (1/0.01–1/0.2), and reached a maximum efficiency of 90% with a Pb-glass/C weight ratio of 1/0.2. However, when Pb-glass/C mass ratio increased from 0.2/1 to 0.3/1, no further efficiency enhancement was observed. Thus, the use of a Pb-glass/C mass ratio of 1/0.2 is more effective and economic for Pb extraction. The efficiency of interfacial reaction is usually influenced by the encountering rate between the reactant molecules (Kushima et al., 2009). More addition of C in the mixture will provide sufficient contact frequency with Pb-glass reactants, which will further enhance the reduction rate. In this section, it was identified that the addition of 20 wt% C to Pb-glass and treatment at 950 °C for 3 min resulted in the best lead extraction performance. Compared to other methods, this method can be conducted in air with a shorter treating time and without

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the facility of a high temperature (above 1000 °C) to obtain metallic lead in the product. Furthermore, direct placement of the mixture of C and Pb-glass into the heated furnace (without the need for slow temperature ramping) is beneficial as it allows for the continuous processing preferred in industrial operations. Therefore, with proper controls, using C as an inexpensive reagent can effectively reduce treatment time and energy, which may allow for a new opportunity to more economically extract lead from waste CRT glass. 3.3. Recovery of Pb with the addition of Na2CO3 To further recover metallic Pb from CRT funnel glass, Na2CO3 was used as a fluxing agent added into Pb-glass/C system for settling down the recovered metallic Pb to the bottom of products during the melting process (Table S6 in SI). Initially, the mixtures were heated at 950 °C (the optimal temperature for lead extraction of Pb-glass/C), but the recovery efficiency of metallic lead was quite low (<20%). The low recovery efficiency may be caused by the reduced encountering rate between the reactant molecules (i.e. C and Pb-glass) and the addition of Na2CO3 in the system. To find out the optimal temperature for recovering metallic lead from CRT Pb-glass/C/Na2CO3 system, the effects of the Na2CO3 dosage (Na2CO3/C of 1/1 to 4/1) and temperature (950–1150 °C) on the lead recovery efficiency were studied and shown in Fig. 5.

Fig. 5. Metallic lead recovery ratios under different thermal treatment times for CRT/C/Na2CO3 samples (mass ratio of Pb-glass/C = 1/0.1) treated at 950–1200 °C for 1 h.

Fig. 4. The extraction (I) and vitrification (II) pathways of metallic Pb by heating the mixtures of CRT Pb-glass and C. Pathway I represents the main reactions for freeing Pb from PbOx polyhedron in glass matrix and the formation of Pb crystals at 600–950 °C for 3 min heating; pathway II shows the re-vitrification of Pb crystal back into silicateglass matrix initiated by higher temperatures (950–1000 °C) and longer heating time (5–120 min).

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Fig. 6. XRD patterns of the residual glass and lead products recovered from C/Na2CO3 of 1/3 products heated at 950–1100 °C for 1 h.

The lead recovery efficiency was firstly enhanced by decreasing C/Na2CO3 to 1/3, and reduced with the further decrease of C/Na2 CO3 to 1/4. Because of the decrease in glass viscosity with the increase of Na2CO3 dosage, the increase in the sedimentation rate of the metallic lead particles was achieved by decreasing C/Na2CO3 from 1/1 to 1/3 (Okada and Yonezawa, 2013; Okada, 2015). However, with further increase of Na2CO3 dosage to C/Na2CO3 of 1/4, the efficiency of interfacial reactions influenced by the encountering rate between the reactant molecules (such as: C and Pb-glass) (Kukukova et al., 2009) was greatly reduced, leading to the reduction of lead recovery efficiency. Thus, the use of C/Na2CO3 mass ratio of 1/3 was more cost-effective for lead extraction. The maximum recovery efficiency of metallic lead was only 20% at 950 °C but increased to 87.3% when the temperature was increased to 1200 °C. Because both glass transition temperature of CRT funnel glass (
480 °C) (Méar et al., 2006a, 2006b) and melting temperature of Pb (327.5 °C) are quite lower than the treating temperature, the Pb-glass and reduced Pb quickly melt into viscous semi-liquid at 950–1200 °C in the pre-heated furnace. The viscosity of the glass decreases with increasing temperature, resulting in an increase in the sedimentation rate of the metallic lead particles (Okada and Yonezawa, 2013; Okada, 2015). However, with the

further increase of temperature to 1200 °C, a slight decrease of the recovery efficiency was observed. As discussed previously, higher temperature would facilitate the transformation of the metallic lead crystals back to the glassy matrix due to the oxidation of lead under high temperature melting (ICER, 2003; Lu et al.,2013). After the reduction melting at 1200 °C, 87.3% of the lead in the Pb-glass was recovered and the residual lead was left in the glass matrix after reduction melting. To confirm the formation of residual lead, the settled lead was separated from the glass samples for XRD analysis. Fig. 6(a) shows XRD patterns of the residual glass treated at temperatures of 950–1100 °C. More residual metallic lead was observed in the samples sintered at low temperatures. The results may suggest that the lower viscosity of glass matrix in 950–1000 °C leads to the decrease of lead sedimentation rate. With the temperature increasing, the reduced intensity of lead signal in the residual glass suggested that higher temperatures (1100–1200 °C) facilitate the settling down of metallic lead, but easily lead to the transformation of metallic lead back into the glass matrix. The schematic diagram for the recovery of metallic lead with the addition of Na2CO3 in Pb-glass/C mixtures was illustrated in Fig. 7. At 950–1000 °C, Pb-glass, reduced metallic lead and Na2CO3 melt into viscous semi-liquid in the pre-heated furnace.

Fig. 7. The reduction (I) and sedimentation (II) pathways for recovering metallic Pb by heating the mixtures of Pb-glass/C/Na2CO3. Pathway I demonstrates the reduction of Pb from PbOx polyhedron and the formation of Pb crystals. By melting CRT Pb-glass and Na2CO3, the highly viscous liquid formed, which inhibited the oxidation of C and metallic Pb and facilitated the formation of Pb crystals. Pathway II reveals the sedimentation of metallic Pb in the bottom of crucible. The melting of Na2CO3 increased the viscosity of the glass liquid and lead to the effective sedimentation of Pb crystals.

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The highly viscous matrix blocks the oxygen supply during the interaction between C and Pb-glass, and is highly effective in inhibiting the oxidation of C and metallic Pb. In addition, the adding of Na2CO3 increased the viscosity of the glass, leading to an increase in the sedimentation rate of the metallic lead particles. However, further temperature increase may facilitate the revitrification of lead back to silicate-glass matrix. To identify the purity of recovered lead, Fig. 6(b) shows the XRD plots of metallic Pb recovered from C/Na2CO3 of 1/3 at temperatures of 950–1100 °C. Both metallic lead and aluminum were found in the recovered lead samples. As the content of Al2O3 in CRT funnel glass was reported to be
3 wt%, aluminum was reduced by C and settled down together with lead at higher temperatures. To obtain the pure metallic lead, the separation of Al from the recovered products or the modified extraction methods for specific extraction of Pb are required. 4. Conclusions Two reaction pathways (extraction and sedimentation) for recovering metallic Pb from CRT glass by thermal reduction method were provided. In extraction pathway, 90.3% of Pb was extracted from PbOx polyhedron in glass matrix by the addition of C. By directly putting the materials into constant-temperature zone, CRT glass quickly melted into viscous semi-liquid, which inhibited the oxidation of C and facilitated the extraction of metallic Pb. In sedimentation pathway, 87.3% of Pb was recovered and settled down in the bottom with the addition of Na2CO3. However, the vitrification of lead back to the glass matrix under the longer treatment time and higher temperature was observed. Thus, the optimal parameters for recovering metallic Pb from CRT glass were suggested. Compared with current methods, this study introduces a lead extraction technique by directly putting the materials into constant-temperature zone without slow temperature ramping (continuous processing), which is highly beneficial for industrial operations. Acknowledgements This research was supported by National Natural Science Foundation of China (Project 41503087, 21637001) and Hong Kong Research Grants Council (Projects 17206714, 17212015, C704414G). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.wasman.2018.04. 010. References Andreola, F., Barbieri, L., Corradi, A., Lancellotti, I., 2005. Cathode ray tube glass recycling: an example of clean technology. Waste Manage. Res. 23 (4), 314–321. Andreola, F., Barbieri, L., Corradi, A., Ferrari, A.M., Lancellotti, I., Neri, P., 2007. Recycling of EOL CRT glass into ceramic glaze formulations and its environmental impact by LCA approach. Int. J. Life Cycle. Assess. 12 (6), 448– 454. Andreola, F., Barbieri, L., 2007. A. Corradi, I. Lancellotti, CRT glass state of the art: A case study: Recycling in ceramic glazes. J. Eur. Ceram. Soc. 27 (2), 1623–1629. Chen, M., Zhang, F.-S., Zhu, J., 2009. Lead recovery and the feasibility of foam glass production from funnel glass of dismantled cathode ray tube through pyrovacuum process. J. Hazard. Mater. 161, 1109–1113. Chen, M., Zhang, F.S., Zhu, J., 2010. Effective utilization of waste cathode ray tube glass-Crystalline silicotitanate synthesis. J. Hazard. Mater. 182 (1), 45–49. De La Torre, A.G., Bruque, S., Aranda, M.A.G., 2001. Rietveld quantitative amorphous content analysis. J. Appl. Crystallogr. 34 (2), 196–202. Dondi, M., Guarini, G., Raimondo, M., Zanelli, C., 2009. Recycling PC and TV waste glass in clay bricks and roof tiles. Waste Manage. 29 (6), 1945–1951.

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Please cite this article in press as: Lu, X., et al. Lead extraction from Cathode Ray Tube (CRT) funnel glass: Reaction mechanisms in thermal reduction with addition of carbon (C). Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.04.010